Illumination systems have a variety of applications, including projection display systems, backlights for liquid crystal displays (LCDs) and others. Projection display systems usually include a source of light, illumination optics, an image-forming device, projection optics and a projection screen. The illumination optics collect light from a light source and direct it to one or more image-forming devices in a predetermined manner. The image-forming device(s), controlled by an electronically conditioned and processed video signal (typically digital), produces an image corresponding to the video signal. Projection optics then magnify the image and project it onto the projection screen. White light sources, such as arc lamps, in conjunction with color wheels have been used as light sources for projection display systems. However, recently, light emitting diodes (LEDs) were introduced as an alternative. Some advantages of LED light sources include longer lifetime, higher efficiency, superior thermal characteristics and better color gamut.
One example of an image-forming device frequently used in digital light processing systems is a digital micro-mirror device (DMD). The main feature of a DMD is an array of rotatable micro-mirrors. The tilt of each mirror is independently controlled by the data loaded into the memory cell associated with each mirror, to steer reflected light and spatially map a pixel of video data to a pixel on a projection screen. Light reflected by a mirror in an ON state passes through the projection optics and is projected onto the screen to create a bright field. On the other hand, light reflected by a mirror in an OFF state misses the projection optics, which results in a dark field. A color image also may be produced using a DMD, e.g., utilizing color sequencing, or, alternatively, using three DMDs, one for each primary color.
Other examples of image-forming devices include liquid crystal panels, such as a liquid crystal on silicon device
In liquid crystal panels, the alignment of the liquid crystal material is controlled incrementally (pixel-to-pixel) according to the data corresponding to a video signal. Depending on the alignment of the liquid crystal material, polarization of the incident light may be altered by the liquid crystal structure. Thus, with appropriate use of polarizers or polarizing beam splitters, dark and light regions may be created, which correspond to the input video data. Color images have been formed using LCOS panels in a manner similar to the DMDs.
Another type of an image-forming device is a high temperature polysilicon liquid crystal device (HTPS-LCD). HTPS-LCD also includes a liquid crystal layer, in which the alignment can be controlled incrementally (pixel-to-pixel), as determined by the data corresponding to a video signal. The liquid crystal layer is sandwiched between a glass substrate and an array of transparent electrodes, thus being adapted for operation in transmission. Typically, at the comer of each HTPS-LCD pixel, there is a microscopic thin film transistor.